The covalent attachment of fatty acid moieties to proteins is a widespread postranslational modification of many viral and cell proteins. Fatty acids have been suggested to be important in receptor assembly, protease protection, growth regulation, morphogenesis, membrane anchorage and membrane fusion. However, the most fundamental aspects of acylation and the role fatty acids play in protein structure and function are not yet understood. One of the best characterized membrane glycoproteins is also an acylprotein; the influenza virus hemagglutinin (HA). Recent studies have implicated fatty acid on the HA in the process of membrane fusion, an event crucial for virus infectivity and a process occurring continuously in eukaryotic cells. I propose to use site-directed mutagenesis of a cloned HA gene and cell-free acyltransferase assays using synthetic peptide substrates to identify fundamental structural requirements for protein fatty acid acylation and to characterize the functional role of fatty acid in membrane fusion and budding. These experiments are designed to test the hypothesis that fatty acid on the HA can significantly influence structural or biological properties of the molecule. The results obtained will increase our understanding of this important human pathogen and also improve our view of a widespread post-translational modification beyond the virological context to many cell proteins.